The present invention relates to an entangled nonwoven fabric of polyester fibers which has improved fire retardant properties and balanced tensile strength properties. More specifically, the invention relates to an entangled nonwoven fabric of polyester fibers which has a fire retardancy of greater than 20 seconds in both the fabric machine direction and cross machine direction when tested in accordance with the standard NFPA Test No. 702.
Entangled nonwoven fabrics have been used for a considerable period of time in many applications. They find use in protective garments used in the operating room and in protective garments used by hazardous material ("hazmat") operators, industrial workers such as paint spray operators, sand blasters and the like. Such fabrics also have found use in surgical drapes and tray covers, wipes, and the like. Many of these uses require that the fabric be fire retardant.
It has long been known to treat textile fabrics so as to reduce their combustibility. Early chemists found that ammonium salts of sulfuric, phosphoric, and hydrochloric acids were effective as fire retardants, as well as certain mixtures of these with borax. Later it was discovered that complex heavy metal ions (stannates and tungstates) improved the water resistance of fabrics treated with ammonium salts. In the 1930's, the effect of mixing antimony oxide with organic halogen compounds was discovered. These three efforts represent the major discoveries on which modern flame-retardant chemicals are based. The technology has become considerably more sophisticated in recent years, but for the most part it represents variations on these earlier themes.
Fire retardancy is often measured by the time it takes to burn a test sample of specified size, with longer combustion times being regarded as indicative of better fire retardancy. The fire retardancy of fabrics comprised of thermoplastic fibers, and more particularly nonwoven entangled polyester fabrics, is attributable in some part to a phenomenon known as "melt off". This means, particularly in a test stand, the thermoplastic fibers melt due to the heat of combustion and drop off the specimen being tested, thus impeding the advancement of the flame front. Latex binders are frequently applied to entangled polyester nonwoven fabrics to enhance dimensional stability; provide abrasion resistance; or to anchor colorants such as pigments. Even if the added latex binder is not flammable in its own right, it tends to restrict the "melt off" phenomenon. This reduces the time it takes to burn the test sample, and the sample is thus regarded, perhaps inaccurately, as having inadequate fire retardancy. Those skilled in the art are always seeking ways to provide fabrics having improved fire retardancy as indicated by increased combustion times in the aforementioned "burn test".
Entangled polyester nonwoven fabrics normally have unbalanced properties, i.e., they have more fibers aligned in one direction (machine direction) as compared to the fibers aligned in a second direction (cross machine direction) which is perpendicular to the first direction. This imbalance causes these polyester fabrics to fail the NFPA Test No. 702 fire retardancy standard even when treated with a fire retardant finish. It has now been found that entangled polyester nonwoven fabrics when treated with a fire retardant finish and cross stretched prior to and while the finish is being dried on the fabric provides a fabric which has a fire retardancy of greater than 20 seconds in both the machine direction and cross machine direction as measured in accordance with NFPA Test No. 702.
It is therefore an object of this invention to provide entangled polyester fiber fabrics of improved fire retardancy.
It is a further object of this invention to provide entangled polyester fiber fabrics which have a fire retardancy of greater than 20 seconds in both the machine direction and cross machine direction as measured by NFPA Test No. 702.
It is another object of this invention to provide fire retardant entangled nonwoven fabrics which have balanced tensile strength properties.
It is yet a further object of the present invention to provide a method of producing the entangled fabric of polyester fibers with balanced tensile strength properties and improved fire retardancy.
As used herein, the term "balanced tensile strength properties" means that the cross direction tensile strength is nearly the same as the machine direction tensile strength.